Heat-sensitive recording materials

ABSTRACT

A heat-sensitive recording material contains, in its heat-sensitive, color-developing layer comprising a leuco dye and an organic acid that reacts with said leuco dye to develop color upon heating, a hydrocarbon compound melting at a temperature in the range from 50° C. to 200° C. and selected from benzylbiphenyls, alkyl-substituted benzylbiphenyls, hydrogenated benzylbiphenyls, hydrogenated alkyl-substituted benzylbiphenyls, hydrogenated terphenyls and hydrogenated alkyl-substituted terphenyls. With its quick thermal response, this material is suitable for high-speed recording.

TECHNICAL FIELD

This invention relates to heat-sensitive recording materials extensivelyused in computer output, printers for desk calculators and the like,recorders for various measuring instruments, facsimiles, automaticticket vendors and heat-sensitive copying machines. More particularly itrelates to heat-sensitvie recording materials with quick thermalresponse suitable for high-speed recording.

BACKGROUND ART

With the spectacular diversification and expanding volume of informationin recent years, various new recording processes and materials have beendeveloped and put to practical use. Of these, the heat-sensitiverecording process has many advantages: (1) desired prints can beobtained simply by heating heat-sensitive materials, with no need forcumbersome development process; (2) heat-sensitive recording materialscan be manufactured and stored more easily and at less costs than anyother type of recording material; and (3) when paper, which is aninexpensive material, is used as the substrate, as is often the case,the resulting heat-sensitive recording materials have a favorable feellike plain paper.

Heat-sensitive recording materials commonly used for this purpose arecomposed of a suitable substrate, such as paper, synthetic paper andsynthetic resin film, and a heat-sensitive, color-developing layercoated thereupon which contains a colorless or faint-color leuco dye andan organic acid that is capable of reacting with said leuco dye todevelop color upon heating.

More recently there has been a great demand in this field for improvedheat-sensitive recording materials adapted for high-speed recordingwhich will give prints of sufficient density with shorter heating timeof, and lower voltage applied to, thermal heads.

In order to meet this requirement, Japanese patent application Laid-openNo. 39,139 (1978) teaches a heat-sensitive recording materialcontaining, in its color-developing layer, at least one compound meltingat a temperature in the range from 60° C. to 200° C. and selected fromalkylbiphenols and substituted biphenylalkanes. Japanese patentapplication Laid-open No. 48,751 (1978) also proposes a heat-sensitiverecording material containing a fusible substance which melts at atemperature in the range from 60° C. to 200° C.

These prior arts, however, are still unsatisfactory in terms of thermalresponse and print quality in high-speed recording.

The primary object of this invention is to provide a heat-sensitiverecording material which shows quick thermal response and hence iscapable of giving sharp and clear prints of high density even inhigh-speed recording. A further object of this invention is to provide aheat-sensitive recording material which contains, in itscolor-developing layer comprising a leuco dye and an organic acidcapable of reacting with said leuco dye to develop color upon heating, aspecific hydrocarbon compound that can act as a color developmentaccelerator. A still further object of this invention is to provide aheat-sensitive recording material having a heat-sensitive,color-developing layer which contains a leuco dye and an organic acidthat reacts, upon heating, with said leuco dye to develop color, saidheat-sensitive, color-developing layer also containing at least onehydrocarbon compound melting at a temperature in the range from 50° C.to 200° C. and selected from the group consisting of benzylbiphenyls,alkyl-substituted benzylbiphenyls, hydrogenated benzylbiphenyls,hydrogenated alkyl-substituted benzylbiphenyls, hydrogenated terphenylsand hydrogenated alkyl-substituted terphenyls.

DISCLOSURE OF INVENTION

This invention relates to heat-sensitive recording materials having aheat-sensitive, color-developing layer which contains a colorless orfaint-color leuco dye and an organic acid that reacts, upon heating,with said leuco dye to develop color, said heat-sensitive,color-developing layer also containing at least one hydrocarbon compoundmelting at a temperature in the range from 50° C. to 200° C. andselected from the group consisting of benzylbiphenyls, alkyl-substitutedbenzylbiphenyls, hydrogenated benzylbiphenyls, hydrogenatedalkyl-substituted benzylbiphenyls, hydrogenated terphenyls andhydrogenated alkyl-substituted terphenyls.

The leuco dyes used in this invention as color developer are compoundswhich are colorless or faint in color at normal temperature and reactwith an acidic substance to develop color upon heating. Typical examplesinclude triarylmethane dyes, such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide; diphenylmethane dyes,such as 4,4-bis-dimethylaminobenzhydryl benzyl ether; fluoran dyes, suchas 7-diethylamino-3-chlorofluoran; thiazine dyes, such asbenzoyl-leuco-Methylene-Blue; spiro dyes, such as3-methyl-spiro-dinaphthopyran; and leuco bases of auramine, indoline andindigo dyes.

The organic acids used in combination with the above-mentioned leucodyes are compounds which react, upon heating, with said leuco dyes todevelop color. Typical examples include phenols and various types oforganic acids, and those which are solid at room temperature and melt orvaporize when heated to 50° C. or higher are preferably employed. Asillustrated examples may be mentioned among others phthalic anhydride,gallic acid, salicylic acid, 4,4'-isopropylidenediphenol,4,4'-isopropylidene-bis (2-chlorophenol),4,4'-isopropylidene-bis(2-t-butylphenol), 4,4'-sec-butylidenediphenol,4,4'-(1-methyl-n-hexylidene)-diphenol, 4-phenylphenol,4-hydroxydiphenoxide, methyl-4-hydroxybenzoate,phenyl-4-hydroxybenzoate, 4-hydroxyacetophenone, salicylanilide,4,4'-cyclohexylidenediphenol, 4,4'-cyclohexylidene-bis(2-methylphenol),4,4'-benzylidenediphenol, 4,4'-thio-bis(6-t-butyl-3-methylphenol),novolak type phenol resin and haolgenated derivative thereof, α-naphtholand β-naphthol.

The hydrocarbon compounds used in this invention, which melt at atemperature in the range from 50° C. to 200° C. and are selected fromthe group consisting of benzylbiphenyls, alkyl-substitutedbenzylbiphenyls, hydrogenated benzyl-biphenyls, hydrogenatedalkyl-substituted benzylbiphenyls, hydrogenated terphenyls andhydrogenated alkyl-substituted terphenyls, serve as a promotor for thecolor-developing reaction. These have lower melting or sublimation pointthan the leuco dye and/or the organic acid, and are also capable ofdissolving the dye and/or acid when heated, thus accelerating thereaction between the two. The alkyl substituents in thealkyl-substituted derivatives of benzylbiphenyls, hydrogenatedbenzylbiphenyls and hydrogenated terphenyls should preferably be alkylsof 1 to 4 carbon atoms, more preferably methyl or ethyl.

Illustrated examples of these hydrocarbon compounds includeo-benzylbiphenyl (mp:56° C.), p-benzylbiphenyl (mp:86° C.),p-cyclohexylmethylbiphenyl (mp:58° C.), o-cyclohexylmethyldicyclohexyl(perhydro-o-benzylbiphenyl) (mp:62° C.), m-cyclohexylmethyldicyclohexyl(mp:65° C.), alkyl derivatives thereof, 1,3-dicyclohexylcyclohexane(octadecahydro-m-terphenyl), 1,4-dicyclohexylcyclohexane(octadecahydro-p-terphenyl), 1,4-dicyclohexylcyclohexane,4-cyclohexyl-1-phenylcyclohexane, 1,4-dicyclohexen-1-ylbenzene,1-cyclohexyl-4-(p-tolyl)cyclohexane, 1,3-dicyclohexyl-5-methylbenzene,1,4dicyclohexyl-2,5-dimethylbenzene,1,5-dicyclohexyl-2,4-dimethylbenzene,1,4-dicyclohexyl-2,6-dimethylbenzene, 1,5-dicyclohexyl-2,3-dimethylbenzene and 1,4-bis(1-methylcyclohexyl)benzene. Thesehydrocarbon compounds may be used alone or in combination.

The suitable amount of these color-developing accelerator may bedifferent depending on the leuco dye and organic acid used incombination, but normally is in the range from 0.5 to 30 parts byweight, per one part of the leuco dye, more preferably in the range from1 to 10 parts. Any known color-developing accelerator may also beemployed in combination.

The heat-sensitive recording materials of this invention may alsocontain various types of additives depending on specific uses. Theseinclude binders which serve to separately immobilize the finelydispersed particles of leuco dye and organic acid, such as polyvinylalcohol (PVA), methylcellulose, hydroxyethylcellulose, polyacrylic acid,casein, gelatin, starch and derivatives thereof; and metal soaps andwhite pigment, such as calcium carbonate, kaolin, clay, talk andtitanium oxide, to improve the whiteness of final products, to givemoderate surface roughness for ease of subsequent noting, and to preventsticking.

These components are coated on a substrate, such as paper and film, inone or separate layers, forming a heat-sensitive, color-developinglayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows graphs illustrating the temperature versus reflectiondensity relationship of the heat-sensitive recording materials describedin Examples 1 through 6 and Comparative Example.

PREFERRED EMBODIMENTS OF THE INVENTION

The following Examples and Comparative Example further illustrate thisinvention.

EXAMPLE 1

(1) Preparation of Liquor A

Crystal Violet lactone (15 parts by weight), p-benzylbiphenyl (50 parts)and 10% aqueous solution of PVA (500 parts) were intimately mixed in aball mill for five hours, giving Liquor A.

(2) Preparation of Liquor B

Bisphenol A (75 parts by weight) and 15% aqueous solution of PVA wereintimately mixed in a ball mill for five hours, giving Liquor B.

(3) Preparation of Heat-sensitive Recording Paper

Liquor A (100 parts by weight) and Liquor B (100 parts) were mixed, andthe solution thus obtained was coated on paper (50 g/m²) and dried,giving heat-sensitive recording paper with a dry add-on of 5 g/m².

EXAMPLES 2 THROUGH 6

Heat-sensitive recording paper was prepared in a similar manner toExample 1 using, in place of p-benzylbiphenyl, o-benzylbiphenyl (Example2), o-cyclohexylmethyldicyclohexyl (Example 3), p-dicyclohexylbenzene(Example 4), 1,3-dicyclohexylcyclohexane (Example 5) and1,4-dicyclohexyl-2, 5-dimethylbenzene (Example 6).

COMPARATIVE EXAMPLE

Liquor A was prepared from Crystal Violet lactone (15 parts by weight)and 10% aqueous solution of PVA (250 parts). Liquor B was prepared frombisphenol A (75 parts by weight) and 15% aqueous solution of PVA (500parts). Liquor A (100 parts by weight) was mixed with Liquor B (200parts), and the solution thus obtained was coated on paper in the sameway as above, giving heat-sensitive recording paper with a dry add-on of5 g/m².

Each of the heat-sensitive recording paper prepared in Examples 1through 6 and in Comparative Example was tested on a stampcolor-developing tester at different temperatures between 60° C. and130° C., and the color density was measured by a reflection densitometer(Sakura Digital Reflection Densitometer, Model PDA-45). The result issummarized in FIG. 1.

As can be seen from this figure, the heat-sensitive recording paperprepared in Examples 1 through 6 show quicker thermal response (sharperrise in reflection density with increase in temperature), compared withthat obtained in Comparative Example.

INDUSTRIAL UTILITY

As is apparent from the foregoing, the heat-sensitive recordingmaterials of this invention contain, in their heat-sensitive,color-developing layer, at least one specific compound (hydrocarbonsmelting at a temperature in the range from 50° C. to 200° C. andselected from the group consisting of benzylbiphenyls, alkyl-substitutedbenzylbiphenyls, hydrogenated benzylbiphenyls, hydrogenatedalkyl-substituted benzylbiphenyls, hydrogenated terphenyls andhydrogenated alkyl-substituted terphenyls), and hence have betterthermal response (sharper rise, in reflection density with the increasein temperature). This makes them capable of giving clear and sharpprints when used in computer output, printers for desk calculators andthe like, recorders for various measuring instruments, facsimiles,automatic ticket vendors, heat-sensitive copying machines and otherlatest equipment which are operated at increasingly higher speeds.

What is claimed is:
 1. A heat-sensitive recording material having aheat-sensitive, color-developing layer which contains a leuco dye and anorganic acid that reacts, upon heating, with said leuco dye to developcolor, said heat-sensitive, color-developing layer also containing atleast one hydrocarbon compound melting at a temperature in the rangefrom 50° C. to 220° C. said hydrocarbon compound being a benzylbiphenyl.2. The heat-sensitive recording materials as defined in claim 1, whereinsaid hydrocarbon compound is p-benzyl-biphenyl.
 3. The heat-sensitiverecording materials as defined in claim 1, wherein said hydrocarboncompound has an alkyl substituent group (or groups) of 1 to 4 carbonatoms.
 4. The heat-sensitive recording material as defined in claim 1,wherein said hydrocarbon compound has methyl or ethyl substituent group(or groups).
 5. The heat-sensitive recording material as defined inclaim 1, wherein said hydrocarbon compound is a partially hydrogenatedhydrocarbon.
 6. The heat-sensitive recording materials as defined inclaim 1, wherein said hydrocarbon compound is a perhydro-hydrocarbon.